The World Cancer Research Fund and American Institute for Cancer Research recommend plant-based diets consisting of a variety of fruits and vegetables, pulses and minimally processed starchy foods that are low in energy. Their report states that these diets may prevent a variety of cancers (and other chronic diseases) because of their inclusion of constituents that are directly protective, or because of the exclusion of constituents commonly found in foods of animal origin.9 Several other recommendations pertaining to diet and lifestyle are made concerning other known or putative risk factors. There are two major research challenges associated with these recommendations and those arising from other expert reports promoting similar guidelines for a healthy diet.
The first challenge relates to characterising the behaviour of nutrients within complex food systems and the interactions between the constituents of those systems with each other and with human tissues. Evidence linking diet to reduced burden of chronic disease weighs heavily in favour of the protective effects of whole fruits and vegetables, consumed as part of a traditional diet, but this is not reflected in research output. A concerted effort should be made to redress the imbalance between whole-food and high dose, single compound research. It is recognised that the very long-term studies required to determine the impact of any particular intervention on morbidity and mortality rates from chronic disease are difficult to fund and to perform. Furthermore, planning of protocol and interpretation of results from complex food interventions, using early bio-markers of disease risk, is not an easy task. However, science is about meeting such challenges, not avoiding them. The biologically active compounds provided by fruits and vegetables are known to have overlapping effects and probably also have synergistic additive and inhibitory effects on each other. These aspects have barely begun to be addressed in either animal or human experimental studies.
The second challenge relates to public acceptance and action. Three out of four Americans believe that there is too much conflicting information about diet and are confused by the 5-A-Day message. There is no universally accepted convention on which foods should be included in health advice on fruits and vegetables. Are dried fruits included, or fruit and vegetable juices and purees? What is the status of frozen, canned and bottled produce? The lack of more precise guidelines allows complacency about present levels of consumption. A study of fruit and vegetable intake in Scotland found that among respondents whose intake of fruits and vegetables was low (less than two portions per day), 55% thought that they were getting enough and already eating 'more'. Providing practical, quantified advice on healthy intakes of foods may help to solve this problem.35 This leads back to the need for research on the public health significance of 'whole' foods, as well as the constituents of those foods, studied within the dietary and cultural environment of specific populations.
Having clear and scientifically supported guidelines, however, does not guarantee compliance. Discussions of approaches to increase fruit and vegetable intake are beyond the scope of this chapter but research on the effectiveness of different strategies is obviously vital if the science and guidelines relating to 'healthy eating' are to be translated into better long-term health.
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